What PIN?

I'm a prepaid customer and wanted to add another device. I went to My Profile and then Create Another Account and bought a prepaid sim and data service for my iPad through the "Bring Your Own Device". Got the sim today and popped it in. When I clicked Manage my T-Mobile Account in the cellular data tab in settings, it asked me to sign in and I did and it asked me to link the device to my T-Mobile ID and asked for a PIN. I used my Tmobile PIN but it said it was incorrect so I called their number and asked for a PIN change for the cellular number linked to the SIM and used the new PIN but it said it was still incorrect. Sorry for the long story but does anyone have an idea of what PIN it could be asking for?